1. Field of the Invention
This invention relates to the fabrication of multilayer ceramic (MLC) substrates adapted for mounting semiconductor devices, and wherein ceramic green sheets are punched with via holes and screened with metallurgical pastes for surface wiring and connections therethrough, in an environment wherein screened sheets are stacked and laminated forming a densely wired structure that is sintered to form the finished substrate. More particularly this invention relates to a system and a method for treating the ceramic green sheet to minimize the residual stresses that develop during sheet casting and drying.
2. Description of related art
High density packaging of semiconductor devices makes possible the mounting of over 100 chips on a single substrate as described in "Ceramic Green Sheet Technology for Glass/Ceramic Multilevel Substrate (ES 9000 System)" by R. W. Nufer, 42nd Electronics Components and Technology Conference, May 18-22, 1992. The Thermal Conduction Module (TCM) comprises 67 individual layers, each punched with over 78,000 via connections, and over 2 million internal wiring interconnections in the finished substrate. The fabrication of these high density requires a very stable green sheet as the basic building block for the MLC substrate in order to achieve precise alignment of all the millions of wiring interconnection.
The green sheet for the TCM is cast from a slurry prepared from a thermoplastic binder and an organic solvent as described in U.S. Pat No. 4,104,345 to R. W. Nufer et al, to create a microporous green sheet that has sufficient porosity to compress during lamination and allow the metallurgical line to penetrate into the green sheet structure providing the sheet with greater than eight percent compressibility. A compressible green sheet, for suitable line enclosure, is achieved with a binary solvent system; methanol and Methylethyl Ketone (MIBK) which governs the slurry rheology during the final stages of sheet drying. After the slurry is cast onto a Polyethylene terephthlate carrier (PET), the methanol which is 3.2 times more volatile then the MIBK, causes a composition shift towards higher percentages of MIBK. This causes greater than 100X increase in viscosity as shown in FIG. 1. The polymer matrix sets in place while a substantial amount of solvent still remains, such that subsequent solvent volatilization causes minimal shrinkage while producing well controlled sheet porosity. Inherent in forming the sheet in this fashion are high residual stresses since the polymer molecular remains in an extended position and can not achieve its normal conformation. High internal stresses is undesirable since subsequent sheet processing where metallurgical pastes are screened onto the surfaces and into the via holes, plasticizes the green sheet, reducing the glass transition temperature(Tg) and permitting the movement and rearrangement of the previously "frozen" structure. This is unacceptable as higher density wiring requires tight dimensional tolerances, placing demands on green sheet stability.
In semiconductor packaging, some packages but primarily multilayer ceramic (MLC) structures are manufactured from ceramic green sheets. During a process of manufacturing of green sheets, the green ceramic material consisting of a sinterable type of ceramic and a labile binder system are solvated with an appropriate solvent to form a well dispersed slurry that is cast and dried, forming the ceramic green sheet. This green sheet serves as the basic building block for the MLC structure as described by A. J. Blodgett et al, IBM Research Development Journal No. 1, (Jan. 1982.). During the formation of the green sheet, a drying process occurs during which the volatile solvents evaporate transforming the slurry into a green sheet having significantly less volume. This drying process introduces drying stresses that remain until subsequent processing when metallurgical patterns and through via holes are screened with metallurgical paste that contains organic components that absorb into the ceramic green sheet. The paste solvents plasticize the green sheet reducing the glass transition temperature, thereby permitting the movement and rearrangement of the previously "frozen" molecular structure. This is unacceptable as higher density wiring requires tighter dimensional tolerance of the green sheet. Accordingly, the requirements of higher density wiring requires more stability of the dimensions of the green sheets.
In one process, ceramic green sheets produced for glass ceramic substrates have been formed using a continuous casting process where a slurry is cast with a doctor blade onto a polyethylene terephthalate (PET), known as (Mylar .RTM. Registered TradeMark) carrier. Then the slurry is dried at an accelerated rate within the continuous caster. The drying of the slurry on the PET carrier causes the formation of drying stresses in the final sheet. These stresses have been minimized in some products by careful control of sheet drying in relation to the separation from the PET carrier. Stress relaxation has in this case been facilitated to some degree by stripping the green sheet from the carrier prior to the removal of all of the solvents. The green sheet is therefore in a partially plasticized state that allows for some stress relaxation. However, total relaxation and stress relief is impractical at this stage since the desirable level of plasticization would prevent separation from the PET carrier and cause physically induced stresses in the longitudinal direction.
Drying stresses are undesirable since they produce dimensional changes in the green sheets during metallurgical screening. The screening instabilities which can be in excess of several mils can be disastrous in maintaining the alignment of the millions of vias in a high density Thermal Conduction Module (TCM) substrate. The stresses in the green sheet cause this movement because the paste and solvent plasticize the green sheets, reducing the glass transition temperature of the green sheets and permitting a movement and rearrangement of the molecular structures of the material in the sheet. Therefore the elimination of these internal drying stresses is desirable, especially as electronic products become more complex with higher density wiring where dimensional tolerances place greater demands on sheet stability.
Stress can be eliminated in a green sheet by solvent stabilization as taught in commonly assigned U.S. Pat. No. 3,953,562 "Process for the Elimination of Dimensional Change in Ceramic Green Sheets" of G. F. Hait and R. W. Nufer et al solvent stabilization is used to eliminate the instability in screened green sheets by temporarily superplasticizing (plasticize and soften but not dissolve) a thermoplastic binder. During the vapor stabilization process the green sheet is exposed to a solvent that will diffuse instantaneously into the sheet. A higher vapor pressure solvent is desired. This superplasticizes the binder transforming the green sheet into a sheet having the appearance of a wet noodle. The superplasticization process reduces the glass transition temperature of the binder substantially below ambient temperatures and allows for immediate stress relaxation. Following relaxation the plasticizing solvent diffuses equally fast from the sheet returning its normal mechanical properties.
The relative evaporation rate of a solvent is a function of its ambient temperature which produces increased vapor pressure as temperature rises. The rate of evaporation and solvent vapor pressure at a temperature of 20.degree. C. is given in Solvents Guide, C. Mardsden, Cleaver-Hume Press Ltd. The higher vapor pressure solvent such as methylene chloride (dichloro-methane, methylene dichloride) with a 349 mm Hg vapor pressure and relative evaporation rate of about 1600 as based on n-butyl acetate being equal to 100 is desirable because of its greater volatility. An example of other less volatile solvents are as follows:
______________________________________ Relative Evaporation Rate Solvent n-butyl acetate = 100 ______________________________________ xylene 70 methyl isobutyl ketone (MIBK) 165 ethanol 230 methyl ethyl ketone (MEK) 572 methanol 610 acetone 1160 ______________________________________
"High-Stability Green Sheet Binder", R. W. Nufer, IBM Technical Disclosure Bulletin, Vol. 21, No. 11 pp. 4489-4490 (Apr. 1979) states, "A polymeric formulation that utilizes polyvinyl butyral resin and a plasticizer, dipropylene glycol dibenzoate, provides the ceramic green sheet with a greatly improved stability, especially after the green sheets have been vapor stabilized with methylene chloride, as shown by the results in the figure.
The general green sheet formulation is as follows:
______________________________________ Amounts ______________________________________ Alumina 92% Polyvinyl butyral resin 6.4% Dipropylene glycol dibenzoate plasticizer 1.6% ______________________________________
The system has improved tensile strength, bond strength and the distinct advantage of having increased compressibility when drying is accelerated on the continuous caster."
U.K. Patent Application GB 2192848 "Conveying Heating Apparatus and Method" of Drew describes an extrudate convoyed and heat-treated on a cushion of heated gas. The method provides heating to a temperature sufficient to set or otherwise dry and extrudate. The temperature mentioned is above 100.degree. C. and preferably above 200.degree. C. The cushion of hot air is created by a number of spaced nozzles angled in the direction of travel. Other stations may be added to treat the extrudate before it is removed from the chamber.
U.S. Pat. No. 4,690,591 of Nagashima et al for "Method and Apparatus for Transporting an Article in Vacuum" describes "apparatus for automatically transporting an article, such as a wafer for producing a semiconductor device, in a vacuum chamber for vacuum treatment of the article." Gas is ejected from inclined nozzles. The system is used to perform operations such as ion etching, using argon gas for dry etching. In the case of CVD treatment, then hydrogen gas or silane gas is substituted for argon gas. In the case of plasma etching, CF.sub.4 is used (transported by argon gas.)
European EP--43457-A: 82.01.13, EP--43457-B: 86.04.23, DE3174452-G: 86.05.28; PR--80.07.03 8OUS-165579 of J. K. Hassan and J. A. Paivanas, commonly assigned, for an air bearing transport system for semiconductor wafers--uses longitudinally spaced holes to achieve effective coupling of axial-radial and coanda flow characteristics. The transporter incorporates at least two pairs of rows of air supply holes as required in the transporting track. The holes in each pair of rows are spaced longitudinally along the track, preferably with a spacing distance which is constant for pairs of holes in the same row at from 12.7 to 22.9 mm. The holes in each row on each side of the track centerline are spaced longitudinally from the holes in each adjacent row by a spacing distance, ranging from one to three quarters of the spacing distance and preferably are equal to half the spacing distance.
The air supplied to support a wafer is fed to the holes by way of air manifolds disposed longitudinally of the track. A long thin passage connects each hole to a respective manifold. The passages are angularly inclined both towards the track centerline and in the direction of transport of the wafer along the track, achieving both propulsion and motional stability of the wafer. The inclination of these passages towards the track centerline is greater the farther the distance of the row of passages from the centerline of the track.